As is well known, a circuit breaker is an automatically operated electro-mechanical device designed to protect a load from damage caused by an overload or a short circuit. A circuit breaker may be tripped by an overload or short circuit, which causes an interruption of power to the load. A circuit breaker can be reset (either manually or automatically) to resume current flow to the load. One application of circuit breakers is to protect motors as part of a motor control center (“MCC”). A typical MCC includes a temperature triggered overload relay, a contactor and a motor circuit protector (“MCP”). The MCP is a specialized circuit breaker that provides instantaneous protection against instantaneous short-circuit events. In the United States, these motor circuit protector devices must meet National Electric Code (“NEC”) requirements when installed as part of a UL-listed MCC to provide instantaneous short-circuit protection.
Mechanical circuit breakers energize an electromagnetic device such as a solenoid to trip instantaneously in response to a rapid surge in current such as a short circuit. Most existing MCPs protect only a limited range of motors, but should avoid tripping in response to in-rush motor currents that occur during motor start-up. MCPs that sense relatively low currents may not be suitable for motors having a relatively low in-rush current because tripping will occur during normal operation of the motor. On the other hand, MCPs that sense relatively high currents may not trip on relatively low current levels such as those corresponding to locked-rotor current levels. Because of their limited operating range, some existing MCPs cannot protect for both relatively low current levels and relatively high current levels. Other existing MCPs that can protect against a wider range of fault currents are very large and their current transformers require large volumes of steel to remain in their linear range of operation.
Some circuit breakers include a current transformer, along with other electrical components, to make up the breaker system. Presently, current transformers used in existing circuit breaker devices are designed to supply power to trip unit electronics, or to sense low current ranges, or to sense high current ranges, and have a limited operating range. Thus, current transformer devices designed to sense low fault currents cannot effectively sense high fault currents. An additional current transformer specifically designed for supplying power to the trip unit electronics must be incorporated into the circuit breaker, increasing its size, complexity, and cost. Similarly, current transformer devices designed to sense high fault currents cannot effectively sense low fault currents.
What is needed is a current transformer system for use in circuit breaker devices that operates over wide current ranges.